The present invention relates generally to workpiece holders, and more particularly to pneumatically actuated magnetic workpiece holders and a method for their use with conventional vacuum-type lifting devices.
Many manufacturers, including automobile manufacturers, incorporate into their factories lifting assemblies which are manually, mechanically, or robotically operated. These lifting assemblies are utilized to move very heavy workpieces, often made from sheet metal or the like, from one operational area of an assembly line to another. Manufacturers are continually endeavoring to make these lifting assemblies as safe, powerful, efficient, and inexpensive to operate as possible.
Lifting assemblies incorporate thereon or are provided with some type of device for holding an article or workpiece. Some of these devices include xe2x80x9chandxe2x80x9d-like gripping devices. While these work in a generally satisfactory manner, such gripping devices are often cost-prohibitive since the gripping device is usually dedicated to one or a relatively few particularly-shaped workpieces. Thus, such gripping devices cannot be used to lift a wide variety of differently shaped workpieces.
Electromagnetic holding devices are able to hold a wide variety of workpieces, and can be quite capable of lifting heavy loads. However, such devices are often larger and heavier than may be desirable. Moreover, they consume large amounts of energy in order to continually magnetize the electromagnet. Consequently, such devices can be quite expensive to build and operate.
Less expensive and safer to use than other lifting devices are vacuum suction-cup holding devices, which are generally used today. These devices typically comprise a lifting assembly having removably connected thereto a suction-cup fitted workpiece holder, and a source of vacuum pressure, for instance a pneumatic supply capable of generating negativexe2x80x94or vacuumxe2x80x94pressure. Such vacuum holding devices are able to hold a wide variety of workpieces, can be used with reduced risk to the operator, and are generally powerful enough to lift desired loads. Unfortunately, vacuum holding devices also suffer drawbacks. In order to adequately hold and lift many workpieces, the vacuum cups must be relatively large. Further, both the suction cups and the surrounding work environment, must be kept free from dust, dirt, and other debris that might compromise the vacuum seal between the suction cup and workpiece. Further costs associated with vacuum holding devices arise from the necessity of maintaining a constant vacuum in order to hold a workpiece. This necessity for maintaining constant vacuum pressure also presents safety considerations, since a failure of the vacuum supply while a workpiece is being lifted poses obvious workplace hazards. Vacuum holding devices are also less desirable for applications where workpieces with curved or irregularly-shaped surfaces must be lifted and held, since vacuum cups require a relatively flat contact surface in order to create an efficient seal.
It would consequently be desirable to provide, with minimal cost, a simple and efficient workpiece holding device capable of securely holding workpieces of varying shapes and sizes.
The present invention addresses and solves the problems discussed above, and encompass other features and advantages, by providing a pneumatically-actuated, magnetic workpiece holder comprising a housing having a contact surface for contacting a workpiece to be held, and a magnet assembly translationally disposed in the housing. The magnet assembly comprises a plurality of permanent magnets arranged so that adjacent magnets are of opposite polarities. The housing is adapted for fluid communication with a pneumatic supply. The magnet assembly is biased towards an operative position, according to which the magnet assembly is sufficiently near the contact surface to exert on a workpiece to be held an attractive force sufficient for holding the workpiece in contact with the workpiece holder, and is further translationally positionable by pneumatic pressure from the pneumatic supply towards an inoperative position, according to which the magnet assembly is sufficiently distant from the contact surface so as to be unable to exert on the workpiece to be held an attractive force sufficient for holding the workpiece in contact with the workpiece holder.
According to one feature of this invention, a polymeric boot or cover is provided for the contact surface to thereby prevent damage to the workpiece being held. The polymeric cover preferably comprises an ultra high molecular weight polymer, most preferably urethane.
According a further feature of this invention, the plurality of permanent magnets are further arranged radially about a central axis. Per yet another feature, the magnet assembly further comprises pole pieces positioned between the plurality of permanent magnets.
The permanent magnets are preferably formed from a rare earth metal, preferably selected from the group consisting of neodymium and samarium cobalt, with neodymium being most preferred. Other magnetic materials, such as ferrite and alnico, may also be used.
The magnetic workpiece holder of this invention is particularly adapted for replacing suction-cup workpiece holders in conventional vacuum lifting devices, and a method of utilizing the present invention in this fashion is taught to comprise providing the workpiece holder with a coupling complimentary to the coupling for such conventional suction-cup workpiece holders, so that the magnetic workpiece holder may be substituted for the conventional suction-cup holder in the vacuum lifting device. Further according to this method, the vacuum supply of the conventional vacuum lifting device, comprising a pneumatic supply capable of alternatively generating positive air pressure, is adapted to provide such positive air pressure for employing the magnetic workpiece holder of this invention.